What the test measures
Air tightness testing — also called air permeability or air pressure testing — measures how much air leaks through a building’s envelope. The more airtight the building, the less heat escapes with it, and your SAP calculation will have assumed a specific figure to achieve its pass. The test is where that assumption gets checked.
The result is q50: the air leakage in cubic metres per hour, per square metre of envelope area, with the building held at 50 pascals — written m³/(h·m²). Two numbers matter:
- 8.0 is the limiting value in Approved Document L — the worst a new dwelling may be and still comply at all.
- 5.0 is what the notional dwelling assumes, which means it is roughly what your SAP will have assumed too.
So 8.0 is the pass mark you are told about, and 5.0 is the one you actually need. A result between the two is legal but has to be paid for elsewhere in the SAP.
Every dwelling, since 2022
The 2021 edition of Approved Document L, in force from 15 June 2022, ended sample testing. Under the previous rules a developer could test a proportion of the plots and assign the rest a penalty value; now every new dwelling in England is tested individually, and the result — including any failure that was later put right — is reported to Building Control. Wales requires the same under its own 2022 edition.
Two methods are approved, both run to CIBSE TM23 (2022): the familiar fan pressurisation test, where a blower door is fitted into an external doorway and the building is pressurised and depressurised at 50 Pa; and the low-pressure pulse method, which works at 4 Pa and has its own limiting value of 1.57. The fan test remains the standard on most sites.
Who can test, and what Building Control gets
Regulation 43 of the Building Regulations says a building control body may accept, as evidence of compliance, a certificate from a person registered with the Air Tightness Testing and Measurement Association (ATTMA) or with Elmhurst Energy. A certificate from a registered tester discharges the requirement on its own — no separate notice needed. The certificate must reach the building control body within seven days of the final test.
Ask any tester which scheme they are registered with before you book — it is the difference between a certificate Building Control accepts as it stands and one they have to query.
Why the result matters more than it used to
Approved Document L requires the as-built SAP to be run “using figures for the building as constructed” — including the measured air permeability. Not the design figure. So a test result is not a certificate to file; it is an input to the calculation that decides whether the dwelling complies, and a poor result has to be absorbed by the rest of the specification. That is why we would rather test early enough to seal and retest than test on the last day.
How to pass first time: the pre-test checklist
Most failures are simple sealing issues, and most are visible before the fan goes in. Before we arrive, check:
- Gaps in and around suspended timber floors, and the floor-to-wall joint (especially timber frame)
- Leaky or poorly sealed windows and doors, and gaps around frames
- The ceiling-to-wall joint at the eaves; gaps around loft hatches
- Service penetrations through ceilings; vents penetrating ceiling or roof
- Kitchen and bathroom wall vents and extractor fans; gaps around waste pipes
- Open chimneys; gaps around electrical fittings in hollow walls
- All boxing sealed; the backs of toilets (especially wall-hung WCs)
What gets sealed for the test, and what doesn’t. The test is run to ISO 9972’s method for regulatory compliance, which draws a line between openings that are sealed and openings that are merely closed. Extract fans, passive vents and mechanical ventilation terminals are sealed, as are fireplaces and woodburners. Trickle vents, windows, external doors and loft hatches are closed but not sealed — taping over a trickle vent makes the result invalid, not better. Water traps need to be filled. And we need the wind to behave: testing is not reliable above about 3 m/s at ground level.
Print it, walk the site and tick each item off before we arrive. Includes the temporary-sealing do's and don'ts. No sign-up.
What we need from you
Drawings. The envelope area is measured to the line that does the air sealing — normally the inside face of the external walls, the ceiling and the ground floor, ignoring internal walls — so we take it from the plans and sections and verify it on site. Get the area wrong and the result is wrong, which is why the tester, not the builder, is responsible for it.
Commercial buildings
Non-domestic buildings are tested to the same method and the same limiting value of 8.0 m³/(h·m²), under Approved Document L Volume 2. Larger and more complex buildings take longer to prepare and may need more than one fan; tell us the floor area and the number of external doors when you ask for a price.
Passivhaus
Passivhaus certification requires n50 ≤ 0.6 air changes per hour at 50 Pa, tested in both pressurisation and depressurisation. That is a different quantity from the Part L q50 — air changes per hour of internal volume rather than leakage per square metre of envelope — and there is no reliable conversion between them, because the relationship depends on the building’s surface-to-volume ratio. We test to both.
Booking
We test dwellings and commercial buildings across the south and beyond, and can bundle testing with your SAP work — see the Compliance Pack. Get a quote or call 01202 623236.
Common questions
Does every new house need an air tightness test?
Yes. Since the 2021 edition of Approved Document L came into force on 15 June 2022, every new dwelling in England is pressure tested individually. The old sampling regime — test a proportion of the plots and give the untested ones a penalty value — has gone. Wales requires the same under its 2022 edition.
What air tightness result do I need to pass?
The legal backstop is 8.0 m³/(h·m²) at 50 Pa — worse than that and the dwelling cannot comply at all. But the number that actually matters is the one your SAP calculation assumed, and the notional dwelling Part L measures you against is set at 5.0. If the design SAP assumed 5 and the test comes back at 7, the as-built SAP has to use 7, and the shortfall has to be made up somewhere else in the building.
What is q50?
The air permeability: the volume of air that leaks through the building envelope per hour, per square metre of envelope area, with the building held at a 50 pascal pressure difference. Units are m³/(h·m²). It is a different quantity from n50 — air changes per hour — which Passivhaus uses, and there is no valid conversion between them because the relationship depends on the building's shape.
Who is allowed to carry out the test?
Regulation 43 says a building control body may accept a certificate from a tester registered with the Air Tightness Testing and Measurement Association (ATTMA) or Elmhurst Energy. A certificate from a registered tester is accepted on its own; an unregistered tester has to demonstrate competence and calibrated equipment separately. Ask which scheme your tester is registered with before you book.
What happens if the dwelling fails the air test?
You find the leaks, seal them, and we retest — the failure and the remedy are both reported to Building Control. Most failures are simple sealing issues found with the fan running, and many are fixed the same visit. The risk is programme, not paperwork: the as-built SAP cannot be finalised until there is a passing result, so leave time before completion.
When should the test be booked?
When the envelope is complete and every planned sealing job is done — windows and doors fitted, service penetrations sealed, loft hatch in, wet trades finished — but before it is too late to get at a leak. The result has to go to Building Control within seven days of the final test, so the test needs to sit inside the completion programme, not at the end of it.
What do you need from me before the visit?
Drawings. The result is expressed per square metre of envelope, measured to the line that does the air sealing — normally the inside face of the external walls, ceiling and floor — so we take the area off the drawings and check it on site. Then the pre-test checklist, walked and ticked.
Which edition of Part L applies to my plots?
Today, the 2021 edition. The 2026 edition is published and takes effect on 24 March 2027 for most work, with transitional cover for work notified before that date and started by 24 March 2028. The testing requirement itself does not go away — it gets tighter.